1. Field of the Invention
The present invention relates to a spin valve sensor with a stable antiparallel pinned layer structure exchange coupled to a nickel oxide pinning layer and, more particularly, to an antiparallel pinned layer structure which has at least first and second antiparallel pinned layers wherein at least one of the antiparallel pinned layers includes a thin film that has positive magnetostriction.
2. Description of the Related Art
A high performance read head employs a spin valve sensor for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or bias point position in response to positive and negative magnetic fields from a moving magnetic medium. The quiescent position is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from a rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry which is discussed in more detail hereinbelow.
The thickness of the spacer layer is chosen so that shunting of the sense current through the sensor and a magnetic coupling between the free and pinned layers are minimized. This thickness is less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. This resistance, which changes due to changes in scattering of conduction electrons, is referred to in the art as magnetoresistance (MR). A spin valve sensor has a significantly higher magnetoresistive (MR) coefficient than an anisotropic magnetoresistive (AMR) sensor. For this reason it is sometimes referred to as a giant magnetoresistive (GMR) sensor. Magnetoresistive coefficient is dr/R where dr is the difference in resistance between minimum resistance, where the magnetic moments of the free and pinned layers are parallel, and maximum resistance, where the magnetic moments of the free and pinned layers are antiparallel, and R is the minimum resistance, where the magnetic moments of the free and pinned layers are parallel.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. Another type of spin valve sensor is an antiparallel (AP) pinned spin valve sensor. The AP pinned spin valve sensor differs from the simple spin valve sensor in that an AP pinned structure has multiple thin film layers instead of a single pinned layer. The AP pinned structure has an AP coupling layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetic moment oriented in a first direction by exchange coupling to the antiferromagnetic pinning layer. The second pinned layer is immediately adjacent to the spacer layer and is antiparallel coupled to the first pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the AP coupling film. Accordingly, the magnetic moment of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetic moment of the first pinned layer.
The AP pinned structure is preferred over the single pinned layer because the magnetic moments of the first and second pinned layers of the AP pinned structure subtractively combine to provide a net magnetic moment that is less than the magnetic moment of the single pinned layer. The direction of the net moment is determined by the thicker of the first and second pinned layers. A reduced net magnetic moment equates to a reduced demagnetization (demag) field from the AP pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net pinning moment, this increases exchange coupling between the first pinned layer and the pinning layer. The AP pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
The first and second pinned layers of the AP pinned structure are typically made of cobalt (Co). Unfortunately, cobalt has high coercivity, high magnetostriction and low resistance. When the first and second pinned layers of the AP pinned structure are formed they are sputter deposited in the presence of a magnetic field that is oriented perpendicular to the ABS. This sets the easy axis (e.a.) of the pinned layers perpendicular to the ABS. During a subsequent making of the magnetic head, the AP pinned structure is subjected to magnetic fields that are directed parallel to the ABS. These fields can cause the magnetic moment of the first pinned layer to switch from a desirable first direction perpendicular to the ABS to an undesirable second direction which is not perpendicular to the ABS. The same occurs to the second pinned layer of the AP pinned structure. If the coercivity of the first pinned layer of the AP pinned structure is higher than the exchange coupling between the first pinned layer and the pinning layer the exchange coupling will not return the magnetic moment of the first pinned layer to its original direction. This ruins the read head. This problem can occur during operation of the magnetic head in a disk drive when a magnetic field stronger than the exchange field of the first pinned layer of the AP pinned structure is exerted on the read head.
Efforts continue to increase the MR coefficient (dr/R) of GMR heads. An increase in the MR coefficient equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. When these efforts are undertaken it is important that the coercivity (HC) of the pinned layer next to the pinning layer not exceed the exchange coupling field therebetween.
The present invention provides a highly stabilized antiparallel (AP) pinned layer structure which is exchange coupled to a nickel oxide (NiG) pinning layer. At least one of the AP pinned layers includes a thin film composed of a material that has a positive magnetostriction. In an embodiment of the invention, where only one of the AP pinned layers has a thin film composed of a material having positive magnetostriction, the AP pinned layer structure is exchange coupled to the nickel oxide (NiO) pinning layer. With this arrangement the AP pinned layer that is exchange coupled to the nickel oxide (NiO) pinning layer has a stress induced uniaxial anisotropy which is oriented perpendicular to the surface planes of the layers of the spin valve sensor. This uniaxial anisotropy promotes a pinning of the magnetic moment of the AP pinned layer perpendicular to the planes of the layers caused by the exchange coupling of the AP pinned layer to the nickel oxide (NiO) pinning layer. The stress induced uniaxial anisotropy of the AP pinned layer is in the same direction as the orientation of the magnetic moment of the AP pinned layer due to the exchange coupling with the nickel oxide pinning layer. Accordingly, when the pinning strength of the nickel oxide (NiO) pinning layer is degraded in a magnetic disk drive with operating temperatures as high as 150xc2x0 C. the stress induced anisotropy, due to the positive magneto striction of the film in the AP pinned layer exchange coupled to the nickel oxide (NiO) pinning layer, maintains the orientation of the magnetic moment of the AP pinned layer perpendicular to the planes of the layers of the spin valve sensor.
In my investigation I found that when the positive magneto striction film of the AP pinned layer interfaces the AP coupling layer, which is typically ruthenium (Ru), the performance of the spin valve sensor is degraded. This has been overcome by providing the AP pinned layer with a thin film of cobalt (Co) or cobalt iron (CoFe) between the positive magnetostriction thin film and the ruthenium (Ru) AP coupling layer. I further found that when the positive magnetostriction thin film is a material other than nickel iron (NiFe) the performance of the spin valve sensor is degraded when the positive magnetostriction thin film interfaces the nickel oxide (NiO) pinning layer. This problem has been overcome by providing the AP pinned layer with a nickel iron (NiFe) thin film between the positive magnetostriction thin film and the nickel oxide (NiO) pinning layer.
In a preferred embodiment the second AP pinned layer, which interfaces the spacer layer, also includes a thin film layer that has positive magnetostriction. This further supports pinning of the first AP pinned layer by antiparallel exchange coupling between the first and second AP pinned layers. In the same manner as the first AP pinned layer the second AP pinned layer may include a thin film of cobalt (Co) or cobalt iron (CoFe) between the positive magnetostriction thin film and the AP coupling layer of ruthenium (Ru). I have further enhanced the performance of the spin valve sensor by providing the second AP pinned layer with a third thin film which distinguishes it from the first AP pinned layer. The second AP pinned layer may be provided with a thin film of cobalt (Co) or cobalt iron (CoFe) between the positive magnetostriction thin film and the copper (Cu) spacer layer. The preferred AP pinned layer structure includes all of the thin films, as described hereinabove, exchange coupled to a nickel oxide (NiO) pinning layer. However, in a broad aspect of the invention only one of the AP pinned layers has a positive magnetostriction thin film, with or without the other thin films, exchange coupled to pinning layers which may be composed of antiferromagnetic materials other than nickel oxide (NiO). The above arrangement may be applied to either a top or a bottom spin valve sensor wherein the pinning layer is at the bottom or at the top respectively of the spin valve sensor. Further, the present invention applies to a single pinned layer structure which includes a positive magnetostriction thin film and a thin film of cobalt (Co) or cobalt iron (CoFe) located between the positive magnetostriction thin film and the copper (Cu) spacer layer. This single pinned layer structure may further include a thin film of nickel iron (NiFe) between the positive magnetostriction thin film and the nickel oxide (NiO) pinning layer when the positive magnetostriction thin film is composed of a material other than nickel iron (NiFe).
An object of the present invention is to provide an antiparallel (AP) pinned layer structure which has a net positive stress induced uniaxial anisotropy that supplements a pinning field between the pinned layer structure and a pinning layer.
Another object is to provide a highly stable antiparallel pinned layer structure that is exchange coupled to a nickel oxide (NiO) pinning layer during operating temperatures of the AP pinned layer structure in a magnetic disk drive.
A further object is to provide one or both of the AP pinned layers of an AP pinned layer structure with a thin film composed of a positive magnetostriction material.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.